Robotic arms are frequently used in automated machining to enhance production efficiency. The tool position is often determined with respect to a machining reference plane. If this plane is not level, machining precision may decrease. However, most researchers have focused on measuring machine tool lengths to improve tool position measurements; few have investigated correcting the levelness of the machining reference plane. In this study, an automatic method for plane correction was developed. A robotic arm equipped with a mechanical probe was used to measure locations on the machining reference plane, and its deviation angle was calculated. These data were then input to the RoboDK software for the correction of the machining reference plane, the accuracy of which was validated by measurements along an S-shaped path over the machining reference plane, and both the flatness and range of the measured points were used as evaluation metrics. Finally, actual engraving was conducted to confirm the quality of the robotic arm machining.